Crystal chemical principles of selective synthesis of bismuth-containing phases with perovskite-like structures

Bismuth-containing phases with perovskite-like structures were systematized and assigned to 11 homologous series. For two phases differing in bismuth coordination, Bi₂A _k ^′ A ₂ ^″ Cu_k+1O_x and Bi₂A _k ^′ Cu_k+1O_x, crystallochemical principles of their formation are defined. The range of existence of these phases was found and confirmed by syntheses. Lomonosov Moscow State Academy of Fine Chemical Technologies. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 5, pp. 34–40, September–October, 1994. Translated by L. Smolina     

Conditions for Titania(IV) Recrystallization from the η Modification to Anatase

The effect of some sensitizers (methylene blue, methyl red, chlorophylls, coproporphyrin I, and hydrogen peroxide) on the transformation of η-TiO₂, which is an anatase precursor in titanyl sulfate hydrolysis, into anatase has been studied. A decrease in the average crystallite size and suppression of the transformation of η-TiO₂ into anatase is observed in the presence of methylene blue and methyl red organic dyes, as well as Ti(IV) peroxocomplexes,. The sample synthesized in the presence of 1 M H₂O₂ is characterized by the smallest crystallite sizes (2?4 nm) and the highest photocatalytic activity.     

Multiple-Band Andreev Transport in Optimally Doped Superconducting Oxypnictides

JETP Letters - Using multiple Andreev reflections (MAR) effect spectroscopy of SnS junctions, we directly determine temperature dependences of the superconducting order parameters, excess Andreev...     

Multiple Andreev Reflection Spectroscopy of Optimally Doped Ba(Fe,Ni)2As2 Superconducting Pnictides

JETP Letters - The current-voltage characteristics and dynamic conductance spectra of the SnS junction in Ba(Fe, Ni)2As2 superconducting pnictide of almost optimal composition have been obtained. A...     

Structural features of bismuth borates in the system <Emphasis Type="Italic">n</Emphasis>Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-<Emphasis Type="Italic">m</Emphasis>B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The crystal structures of general composition nBi₂O₃-mB₂O₃ were analyzed and systematized with the use of the structures of borate groups. Based on the CNs calculated by the bond valence method, the shapes of bismuth coordination polyhedra derived from an octahedron were suggested. A correlation was found between the number of BO₃ triangles and BO₄ tetrahedra in borate groups, the average CN of Bi atoms, and the degree of distortion of Bi polyhedra as a function of the m: n ratio, as well as between the polarity of BO₄ tetrahedra and noncentrosymmetry of the structures. The role of Bi³⁺ with the active E pair in the manifestation of specific features of the forms of bismuth polyhedra and the types of connection of boron polyhedra was elucidated.     

Peculiarities of the microstructure of a nanoscale modification of η-TiO2

Samples with nanoscale η-TiO₂ phase have been obtained by sulfate and modified sulfate methods and are characterized by a complex of techniques: X-ray diffraction, electron diffraction, small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy, and X-ray energy-dispersive analysis. The effect of sample formation conditions on the size and shape of crystallites, content of the amorphous component in the samples, and their elemental composition has been established. A significant change (depending on the synthesis conditions) in the parameters of the diffraction reflection with d ～ 17–21 Å (intensity and interplanar spacing d, Å), pronounced for η-TiO₂, is revealed. This change is most likely related to the variation in the content of water molecules in the interlayer space of η-TiO₂ structure and/or the change in crystallite shape.     

Doping influence on Sm1 − x Th x OFeAs superconducting properties: Observation of the effect of intrinsic multiple Andreev reflections and determination of the superconducting parameters

We studied SNS and S-N-S-N-...-S contacts (where S is a superconductor and N is a normal metal) formed by “break-junction” technique in polycrystalline Sm_{1 ? x} Th _x OFeAs superconductor samples with critical temperatures T _C = 34–45 K. In such contacts (intrinsic) multiple Andreev reflections effects were observed. Using spectroscopies based on these effects, we detected two independent bulk order parameters and determined their magnitudes. Theoretical analysis of the large and the small gap temperature dependences revealed superconducting properties of Sm_{1 ? x} Th _x OFeAs to be driven by intraband coupling, and $\sqrt {V_{11} V_{22} } /V_{12} \approx 14$ (where V _ij are the electron-boson interaction matrix elements), whereas the ratio between density of states for the bands with the small and the large gap, N ₂/N ₁, correspondingly, was roughly of an order. We estimated “solo” BCS-ratio values in a hypothetic case of zero interband coupling (V _{i ≠ j} = 0) for each condensate as 2Δ_{L, S}/k _B T _C ^L,S ≤ 4.5. The values are constant within the range of critical temperatures studied, and correspond to a case of strong intraband electron-phonon coupling.     

Colors of mixed-substituted double molybdate single crystals having scheelite structure

X-ray diffraction, neutron diffraction, optical and absorption spectroscopy, electron probe microanalysis, and temperature-dependent electrical conductivity measurements were used to study structural features and defect generation to be responsible for coloring of single crystals of nominal compositions (Na_0.5La _x Gd_{0.5 ? x} )MoO₄:Tm having scheelite structure which were Czochralski grown under a weakly oxidizing atmosphere followed by air annealing. In nonannealed crystals, the color was found to be due to oxygen vacancies and their compensator electrons (black color) and, most likely, electrons located on Mo6+ ions (lilac color); the concentration of these defects decreased upon air annealing, thereby discoloring the samples. The yellow color of crystals was interpreted as arising from possible formation of a color center (an association of a molybdenum vacancy and a hole).     

Scale effect of Young's modulus of highly oriented polymers

The scale effect of the acoustic Young's modulus E of oriented polymer fibers has been experimentally revealed. If the fiber length L is smaller than its critical value L₀, the modulus decreases proportionally to the logarithm of the fiber length. An increasing temperature causes a proportional increase in the slope dE/d(In L). The scale effect is explained by the influence of the specimen volume on the probability of initiation of intense thermal fluctuations, which cause a decline in the Young's modulus.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 6, pp. 839–846, November–December, 1998.     

Synthesis and properties of azoles and their derivatives

Thermal condensation of 1-cyanoadamantane with arylhydroxamic acid chlorides gave 5-(1′-adamantyl)-1,2,4-oxadiazoles containing aromatic groupings in the 3 position. The same 1,2,4-oxadiazoles were synthesized by reaction of the N-oxides of the appropriate nitriles with adamantane-1-carboxylic acid methyl imino ester obtained from 1-cyanoadamantane. The corresponding 5-(1′-adamantyl)-3-substituted 1,2,4-oxadiazoles were obtained by condensation of the latter with amidoxines.